I don't think they pick who is called based on what they're wearing. It's done by random number selection, no? If I wasn't called each day l had to go back to work so I wore normal office attire but there were people there wearing all sorts incl trainers.

I got picked recently and it was names out of a bowl. I was wearing work clothes (as I'd hoped not to be picked and be able to head into work) - and in the end the case got dropped very quickly so I was back into work after lunch. Most people there were either in work clothes or just reasonably smart (defendant in trackies though). I wouldn't worry about trainers at all.